Gamma-ray measurements serve to detect radioactivity, whether it is from natural background radiation or from artificially created and deployed sources. They are used in many industries, such as non-destructive testing and inspection of materials and equipment, medicine, geologic (i.e., mineral) exploration, as well as in oil and gas exploration.
In oil and gas exploration, gamma-ray measurements can be used to determine the properties of formations surrounding wells. Gamma ray tools are used to measure density, porosity, photoelectric factor (Pe), hydrogen index, salinity and thermal neutron capture cross section (sigma).
Gamma ray detectors generally comprise a scintillation detector, composed of a scintillation crystal, e.g. Nal, BGO, Csl, GSO or LSO, coupled to a photomultiplier tube, see for example U.S. Pat. No. 6,467,781.
U.S. Pat. No. 6,400,088 describes an infrared photon detector that comprises an array of nanotubes that absorbs infrared photons and creates photoelectrons. Infrared photons constitute electromagnetic radiation, as do gamma rays; however, infrared photons contain an energy that is several orders of magnitude lower than the typical gamma-ray spectrum.
US20030025084 describes a radiation detector containing carbon nanotubes. The nanotubes are mounted on a scintillator and used to convert the emitted light to electrical energy. The visible light from the scintillation screen is only one order of magnitude (a factor ten) more energetic than the infrared radiation; it is still several orders of magnitude lower than the gamma-ray spectrum.
As the diameter and length of nanotubes are in the order of a few nanometers to several nanometers, their use in gamma ray detectors allows for the development of smaller devices. Nanotube based detectors also have an advantage over crystal based scintillation detectors in their ability to withstand high temperatures and maintain stability in such conditions. Therefore the object of the invention is to develop smaller gamma ray detectors that can be used downhole in oil or gas wells.